Those conventionally known methods for obtaining direct positive images are classified into two types: One type is such that a silver halide emulsion in advance having fog specks is used, and the fog speck or latent image of the exposed area of the emulsion is destroyed by utilizing the solarization or herschel effect, and then developed to thereby obtain a positive image. And the other is such that an internal latent image-type silver halide emulsion not subjected to fogging treatment (generally, surface-fogging treatment) until the time of imagewise exposure is used, and the emulsion, after being imagewise exposed, is subjected to fogging treatment (core-making treatment), and then surface-developed, or the emulsion, after being imagewise exposed, is surface-developed while being subjected to fogging treatment, whereby a positive image is obtained.
The above-mentioned fogging treatment may be effected by having the emulsion overall exposed, chemically treated by using a fogging agent, developed by using a strong developer solution or thermally treated. The foregoing "internal latent image-type silver halide photographic emulsion" implies a silver halide photographic emulsion comprising silver halide crystal grains each having mainly thereinside a sensitivity speck so that a latent image is formed inside the grain as a result of being exposed to light.
Of the aforementioned two methods of forming positive images, the latter method, as compared to the former, generally results in a high sensitivity, so that the latter is suitable for use in the high sensitivity-requiring photography.
In this technological field, various techniques have hitherto been known: For example, those methods are known which are described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,761,266, 3,761,276 and 3,796,577, and British Pat. No. 1,151,363.
The mechanism of forming positive images are considered as follows, as described in, e.g., the Photographic Science and Engineering, Vol. 20, p.158 (1976): The photo electrons generated inside silver halide crystal grains by imagewise exposure are selectively captured into the inner part of the grain, whereby an internal latent image is formed. Since this internal latent image functions as an effective capture center for the electrons in the conduction band, in the case of exposed grains, the electrons that are injected during the course of the subsequent fogging-development are captured into the inner part of the grain to thereby intensify the latent image. In this instance, the latent image, since it all is in the inner part, is not developed. On the other hand, in the unexposed grains, at least a part of the injected electrons is captured onto the surface thereof, and these grains are developed by surface developement.
By using the foregoing techniques of the prior art, light-sensitive photographic materials for the formation of positive images can be produced. However, in order to make these light-sensitive photographic materials applicable to diverse photographic fields, they are required to be even more improved on the photographic characteristics thereof.
For example, as is disclosed in U.S. Pat. Nos. 3,761,267 and 3,206,313, by the chemical sensitization of the inner part of silver halide grains or by using a polyvalent metal ion-doped core/shell-type emulsion, a higher sensitivity can be obtained. Emulsions of this kind, however, have the shortcoming that they result in a low image density. Also, U.S. Pat. No. 3,761,276 discloses chemical ripening treatment to some extent of the surface of silver halide grains in order to remove the above shortcoming that the image density is low, but this treatment is disadvantageous in respect that it increases the minimum image density, significantly deteriorates the silver halide emulsion's stability in the preservation over a long period, and also deteriorates the emulsion's manufacturing stability.
On the other hand, the silver halide emulsion comprised principally of silver chloride disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 32820/1972 produces a positive image of which the maximum density is relatively high, but the minimum density is not adequately low, so that the image is not clear.
Accordingly, in order to make a practical reality of a direct positive light-sensitive material comprising the above-mentioned internal latent image-type emulsion, the internal latent image-type emulsion is desired to be one having an adequately high maximum density and an adequately low minimum density, and being excellent in the preservability.